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This is not implemented using a `def head()` in the view, as the HEAD request
should return all headers that the GET request would send. Hence,
updated the server logic to provide a proper HEAD request.

This assumes most people are on nginx versions newer than 1.5.9

Newer versions of nginx require a properly encoded URI in the X-Accel-Redirected response. According to research done by people on the django-sendfile issue tracker, the specific version number seems to be 1.5.9.

The ``PRIVATE_STORAGE_NGINX_VERSION`` setting is used to determine the proper encoding (defaults to current behaviour of not quoting).

https://github.com/adnrs96/django-sendfile/commit/4488e520f04661136e88be38281e789ae50a260c

The serve method of the DjangoStreamingServer class support
If-Last-Modified, and for this it requires a timestamp of the last
modified time of the private file. Python 3 datetimes have a
timestamp method, which is used to generate the required timestamp,
but this isn't available in Python 2. Serving files on S3 therefore
fails if Python 2 is used.

This commit adds an alternative means of generating the timestamp
if Python 2 is used.

Update documentation on sending files, given that ``wsgi.file_handler`` support lives in Djagno 1.8+

This is needed to fetch content from S3 without needing a full_path